Instruments for recording acoustical and seismic signals at the ocean floor have been in use for many years. The traditional use for these devices was in the field of earthquake monitoring. The earthquake devices are concerned primarily with first arrival time of the seismic wave, rather than any component characteristics of the seismic wave. These devices are typically refereed to as "OBSs", i.e., Ocean Bottom Seismometers.
The measuring and recording of seismic signals is of particular concern in oil exploration geophysics and petroleum reservoir development research. For these uses, full wave information is desired, i.e., both the horizontal and vertical components of shear waves and the compressional wave information need to be recorded separately. Prior art has centered on three main groups of apparatus to measure seismic signals on the seafloor. The first type is a cable that contains geophones, which is laid on the ocean floor. This has not worked well, since the cable and geophones are not rigidly coupled to the sediment on the ocean floor, and thus horizontal motion other than that due to the sediment can cause erroneous signals. Even if a large array of sensors per channel is employed, poor sediment coupling leads to poor data. In addition, a two-ship operation will be required, as a specially equipped vessel is necessary for cable deployment in addition to a source vessel.
A second type of recording method is a hydrophone that is anchored to the sea floor. The device digitizes the signals and then transmits them to the surface (a recording vessel). Only one sensor per channel is available, thus no noise-cancelling arrays are possible. There is also no shear-wave detection capability. The method is also very expensive (approximately 10 times a conventional survey cost, and radio interference may preclude its use in some areas.
The largest drawback to anchored hydrophones, however, is the severe depth limitation (about 70 feet). Modern offshore petroleum exploration frequently requires working at depths of 5,000 feet and more.
The third type of seismic recording device is known as Seafloor Seismic Recorders (SSR's). These devices contain the geophones in a sealed package, and record a signal on the seafloor. Data is retrieved by retrieving the apparatus from the seafloor. A retrievable device is re-usable in theory, and is thus extremely cost effective, as they cost over $20,000. SSR's allow 3-component geophone recording, in addition to special configurations using accelerometers or other sensing devices. Excellent coupling to the sediment is achievable with the present invention, due to its novel design. There is also no depth limitation. Prior SSR's have been used in water up to 3,000 meters deep. Prior art SSR's have been hindered by both unreliable retrieving methods and fidelity of data collection. Data collection quality has been reviewed by G. W. Sutton et al., "Lopez Island Ocean Bottom Seismometer Intercomparison Experiment", HIG-80-4, Hawaii Inst. Geophys., Honolulu, 272 pp. (1980) and G. H. Sutton et al., "An Overview and Results of the Lopez Island OBS Experiment", Marine Geophys. Res. Vol. 5, pp. 3-34 (1981).
Data quality is influenced by noise produced by wave motion acting on the apparatus, along with other undesirable energy inputs, imperfect coupling of the geophones to the ocean bottom, limited dynamic range of the apparatus, limited data storage capacity, and overall unreliability. In trying to solve one problem, the solution often exacerbates another problem. For example, one solution to the problem of recovery is to tether the apparatus to a surface ship or buoy. Unfortunately, the action of ocean currents on the tether and waves and wind on the ship or buoy impart large amounts of energy to the geophones, giving poor results.
Four U.S. patents have been assigned to Mobil Oil Corporation. U.S. Pat. No. 4,441,537 discloses an electronic system which releases an on-bottom seismometer, for subsequent recovery. U.S. Pat. No. 4,666,338 discloses an apparatus for retrieving an ocean bottom seismometer, using inflatable flexible housings. U.S. Pat. No. 4,780,863 discloses an apparatus for supplying the power requirements of an ocean bottom seismometer. U.S. Pat. No. 4,692,906 discloses an ocean bottom seismometer that is weighted at the bottom, and has a seawater ballast which may be ejected to provide positive buoyancy for retrieval.
None of the four patents referenced above even attempts to solve the problem of noise that is produced by wave or current motion and the problem of imperfect coupling of the geophones to the ocean bottom. Seismic data results from two components of the seismic signal, a horizontal and a vertical. The horizontal component is usually much fainter than the vertical. When forces other than from seismic vibration act on the geophone, a phenomenon known at "cross coupling" occurs. Specifically, horizontal signals not the result of seismic energy but instead from extraneous sources act to mask the true seismic horizontal signal. This can result in horizontal signals which look like vertical signals. Additionally, noise can mask the first arrival point of the seismic wave. The extraneous energy causing these problems can be imparted to the geophones by at least two major contributors. First, if the geophone mountings are solidly connected to a seismic seafloor recorder chassis, even by a hinged linkage, any forces which act on the chassis will be imparted to some degree to the geophones. The forces which act on the chassis can vary from ocean current forces to the seismic vibrations from the seismic source itself. Second, ocean currents can act directly on the geophone housing. Additionally, if the geophones are not firmly affixed to the ocean floor, horizontal forces will cause friction (and thus signals) between the geophone package and the seafloor. There currently exists no design of a SSR that directly addresses and solves these problems.
An apparatus attempting to accomplish the objectives of this invention was disclosed by Byrne, et al. in "Marine Geophysical Researches" Vol. 5, No. 4, pp. 437-449 (1983). Byrne et al. attempt to achieve minimization of energy input resulting from currents acting on the chassis of the OBS (as they called it) by detaching the geophones from the chassis, except for two small cables. This did not completely solve the problem, as coupling with the ocean floor did not occur, and the geophone assembly was left laying loosely on the ocean floor (as opposed to being coupled to the sediment of the ocean floor), and thus was subject to currents on the ocean floor. Byrne did propose a method to increase contact between the ocean floor and the geophones by placing the geophones in a flexible bag weighted with pellets. This would certainly improve the physical contact between the geophones and the ocean floor, but there are still problems: 1) The bag still lays on the floor and has a profile above the floor, and is thus subject to currents; 2) the coupling is better, but not direct, and any junctions between the seafloor and the geophones will degrade signal quality; and 3) Byrne does not suggest how this additional bulky bag would be deployed, and how it might affect recovery of the OBS.
Another method that is known in the art and attempts to couple the geophones to the sediment has involved the use of spikes. In this method, the geophone was mounted on a heavily (100-200 kg) weighted spike and dropped in the water. The mass was sufficient to drive the geophone into the sediment. Unfortunately, these devices suffer from mass resonance effects. That is, the seismic wave moves the whole mass which vibrates, imparting energy to the geophones, creating erroneous signals.
An ocean bottom seismometer was disclosed by Herber et al. in "Bollettino di Geofisica" Vol. 23, N. 90-91, pp. 233-242 (1981). The system is connected to a surface buoy via a polypropylene line. However, the geophones are not coupled to the sediment adequately to provide the needed data resolution. Because of the cable (line) and the fact that the geophones are attached to the main body, the apparatus is subject to the ocean currents and waves, therefor causing undesirable noise.
Sutton and Duennebier published an article entitled "Optimum Design of Ocean Bottom Seismometers" in Marine Geophysical Researchers, Vol. 9, pp. 47-65 (1987). Experiments indicated that OBS packages should be designed with (1) the minimum mass possible, (2) radius of area in contact with the sediment proportional to the cube root of the mass, and the maximum radius less than 1/4 of the shear wavelength, (3) density of the OBS approximately that of the sediment, (4) a low profile and a small vertical cross section with water, and (5) low density gradients, and a maximum symmetry about the vertical axis. Sutton and Duennebier, however, do not disclose any attempt to actually design an apparatus which solves the problems of insufficient coupling, the effect of ocean currents on the main body, and maintaining recoverability.
Nakamura et al. (including Applicant Donoho) describe an ocean bottom seismograph that was developed at the University of Texas, in "Geophysics, Vol. 52, N. 12, pp. 1601-1611. The device records 3-component data, however, the geophones are not adequately coupled to the sediment. Also, ocean currents act on the main body, which causes undesirable noise and a decrease in data quality. The dynamic mass of the apparatus, as described by Sutton and Duennebier, supra on p. 51 and at FIG. 5 affects the quality of the data as well. In addition, as the recording device is coupled to the geophones, a temporary data storage means is needed.
The prior work is limited in the attempts at recording seismic data useful in petroleum exploration, in relatively deep water, from geophones coupled to the sediment for the reasons set forth above. There is, therefore, a need for an accurate, reliable, code retrievable device that record such seismic data.